Certain optical instruments can be used for measuring refractive index of a substance, or for determining a derivable quantity as based on its dependence on the refractive index. As an example of such a derivable quantity, the measurement of concentration of a component in a medium substance can be made as based on the refraction index measurement. Other derivable quantities comprise density, content of other substances, conductivity, etc. Such instruments utilizing the same principle may apply in suitable part also for other mediums than liquids, i.e. one can determine refractive index also for measuring gases, liquids and transparent solid substances. Liquids can be oils, water-based liquids, solutions thereof etc. Solids can be glass, diamonds or translucent solids.
An ensemble of known measurement procedures are based on the Snell's law and utilization its predictions on the critical angle (αc) of total reflection from a boundary surface between two optically different media (refractive indexes n1 and n2). In such set-ups the boundary surface is formed by the window at the medium whose refractive index is about to be measured.
FIG. 1A illustrates a setup for determination of a refractive index of a medium as based on utilization of the Snell's law.sin(αc)=n1/n2=n  (1)
The light source in FIG. 1A is illustrated by a LED. The light thereof is directed to the boundary surface between the medium S and the measurement prism P forming the window there between. In FIG. 1A the prism sides act as mirrors for bending the path of light rays, whose directions are illustrated by the arrows. The reflected rays of light (from the boundary) form an image ACB, where the C represents the position of the incoming rays at the corresponding critical angle to the detector. The rays arriving to part A are totally reflected from the boundary to the detector but the rays at the B are only partly reflected or scattered, but also partly refracted into the medium S. Thus, the position of the shadow edge C between the light area A and the dark area B indicate the value of the critical angle for total reflection and thus the refractive index can be calculated from the value used as estimate, to be used as such or for determination of a derivable quantity such as concentration component of the medium S if the medium were a liquid.
In such a measurement, when the component concentration in the medium S changes, also the place of the shadow edge changes consequently. In a case of a low concentration of the medium component with a refractive index, dependent on the concentration, the light area at A is larger than the area at B, and in the case of high concentrations vice versa. When the concentration changes, also the position of the C changes.
The shadow edge C can be detected by an imaging element such as CCD-element for instance. Such an optical instrument is disclosed in further detail at www.kpatents.com/pdf/downloads/pr-23.pdf.
Broad-band source for poly-chromatic light allows a continuous measurement of the refractive index. The wavelength of the reflection band edge is measured by noting the wavelengths where a sudden change in spectral intensity occurs and with the index of the prism sensor and the angle of incidence to the prism face known, the index of refraction of the substance is determined also from the Snell's law.
However, the intensity I of reflected light at a boundary surface between the two media is a function of wavelength λ and incident angle α,I=I(λ,α,n),  (2)where n is the relative index of refraction (i.e. n1/n2) defined by the two media at the boundary. The relative index n is a function of the temperature n=n(T). In addition, dependencies may occur also from other, environmental quantities, but as they may be not significant, are not further considered here for simplicity reasons.
In known techniques applying polychromatic light, the dispersed intensity of the light from the boundary into the critical angle of the total reflection in the medium is to be measured with a constant α, angle of incidence. Dispersion occurring at the boundary surface between the prism and the sample, i.e. is utilized as the measurement device measures the dispersion of the critical angle of total reflection in the medium to be measured.
In such known techniques, the measurement utilizes a single ray of polychromatic light with a constant incident angle. It is utilizing the effect, in which part of the spectrum has a total reflection, part has not. In the attached drawing FIG. 1B and in the table below (where αc is the critical angle, angle of total reflection) we can see for the Blue-to-Red spectrum, that the shift of the critical angle αc is of the order of 1 degree. If used for concentration measurement, with a typical full range 0%-100% concentration it corresponds to an angle of the order of 12 degrees, we can see that the maximum obtainable measuring range is limited to 1/12 or +/−4% concentration. This may be sufficient for some applications, such as measurement of salinity in seawater, which naturally is characterized by small fluctuations. Such a narrow range is not sufficient for a wide spread utilizable industrial instrument. The dashed line illustrating the blue (B) wavelength is dashed, as in the illustrative example of FIG. 1B some blue light (B) is reflected partially, and partially refracted into the medium.
λαnm0%100%Blue (B)486.150.36962.283Yellow (Y)589.350.87963.137Red (R)656.351.16863.453
Thus, there is a need to simplify the structure of optical instrument as well as to gain simultaneously more versatile and durable instrument structure that does not need back-up as often as the known devices, but would have a sufficiently wide range applicability for the measurement. Especially there is a need for such a simple instrument for use in fire sensitive and/or explosive gases/vapors containing environments.